Magnetic imaging

ABSTRACT

A composite recording medium comprises a photoconductive layer, a plurality of interdigitated first and second electrodes in electrical contact with the photoconductive layer, and a magnetic recording layer in heat-transfer relationship with the photoconductive layer. Methods and apparatus for magnetically recording an image with the assistance of this composite recording medium are also disclosed.

346/74 MT, 74 TP; 96/] R, l E

O United States Patent 11 1111 3,815,987 Duck et al. I 1' June 11, 1974 MAGNETIC IMAGING [56] References Cited [75] Inventors: Sherman W. Duck, Alhambra; UNITED STATES PATENTS Frederick J. Jeffers, Altadena; 2,798,959 7/1957 Moncrieff-Yeates 96/] E James U. Lemke, Del Mar, all of 2,836,766 5/1958 Halsted 96/].5 Calif, 3,485,62l 12/1969 Kazan 96/l [73] Ass1gnee: glell & I-Iowell Company, Ch1cago, Primary Examiner Robert P Greiner Attorney, Agent, or Firm--Luc P. Benoit [22] Filed: Dec. 27, 1972 211 App]. No.: 318,809 1 ABSTRACT A composite recording medium comprises a photo- I Related Apphcatlm? Data conductive layer, a plurality of interdigitated first and [62] of second electrodes in electrical contact with the photo- I conductive layer, and a magnetic recording layer in [52] Cl 355/3 355/16 355/17 heat-transfer relationship with the photoconductive 96/ l 96/1 346/74 346/74 M layer. Methods and apparatus for magnetically recordg ing an image with the assistance of this composite recording medium are also disclosed.

7 Claims, 3 Drawing Figures MAGNETIC. IMAGING This is a division of :application'Ser. No. 29,584, filed Apr; 17, 1970.

BACKGROUND OF-THE INVENTION 1. Field of the Invention The subject invention-relates to magnetic recording and, more particularly, to the; magnetic..recording- ,of

images with the. assistance.of'thermalgradients..

2. Description of the -PriorsArt Magnetic imagingehas'been 'thesubjectof serious in-: vestigation in recentyears, since-it has-several..advantages over more conventionalimagi-ng techniques.

For instance, magneticimagingoffersthe.prospect of Unfortunately, magnetic .imaging:techniques which have become publically known "tordate cannot compete in terms of light sensitivity and exposurespeedwith photographic methods or-even withelectrostatic xerogf; raphy.

SUMMARY OFTHE-LINVE'NTION The subject inventionovercomesormaterially alle,vi-- ates the above:mentioneddisadvantages and, fromrone;

aspect thereof, resides inra-method ,ofmagnetically recordingan image; comprising-jrrcombination"thestepsv of providing a pluralityof interdigitatedtjfirst and sec-.

ond electrodes; providingt a.photooonductivevlayer in electrical contact -withvtheinterdigitatedfirst and sec-v 0nd electrodes, andpprovidingaa magnetic, recording layer in heat-transferrelationship with'said photoconductive layer. This method furtherincludesthesteps of applying electrical energy to the.interdigitatedfirst and second electrodes and exposing; the photoconductive. layer to the image so asto providetathermalpattern" correspondingto the image; and/.changing theastate.of. magnetization of the recordinglayer in responsesto. the thermal pattern so as to' produce-a;magneticrecordof' the image.

' By way of example, the: interdigitated first and secr ond electrodes may .bealocated. b'etweenvithe:photocom ductive layer'and the-magnetic: recording layer; This arrangement has the; advantage. that: the; electrode. structure does not obscurev thetimage-exposed *tOPySlll" parted to the electrodes.isureadily.transferredto the. magnetic recording-layer:

Alternatively, ,at' least. part of: the, photoconductive.

layer maybe locatedtbetweenxtheinterdigitatedgfirst. and second. electrodes and: the; magnetic.- recording; layer. A preferredxembodiment :ofi:the. invention"ina accordance with thlS"aitCIHZtlVCJSOlUtlOII. h'aSIhQJPhDIOf conductive layer -providem-withna.first:sideqlocatedgat the plurality. of interdigitated? first: ands; secondi elec: trodes, and :with' a second.gsidezoppositesuch :first ,side.

The magnetic recordingt layeris then Pl'OVidfidfifllh6fit-ltransfer relationship: with?" the: photoconductive. layer through'the latter secondisidesof the photoconductive layer.

The. latter alternativeqsolution"is presently. preferred because available. techniques-favor. the.:provision-of an electrode structure ona substrate over the provision of.

an electrodestructure on a photoconductive layer, and becauseproblemsof light absorption by the photoconductivelayer are alleviated if theelectrodes are at the image-exposed side.of the photoconductive layer.

From another aspect thereof, the subject invention resides in a method of magnetically recording an image on a composite recording medium, which medium includes a photoconductive. layer, a plurality of interdigitated firstand second-electrodes in electrical contact withthe photoconductive layer, and a magnetic recording layer in heat-transfer relationship. with the photoconductive layer. This method comprises in combination the .stepsof applying electrical energy: to the interdigitated firstand second electrodes and progressively exposing the photoconductivelayer to the image in a direction progressing parallel tothe interdigitated electrodes sov as to produce a thermal pattern corresponding to the image, and changingthe state of magnetization of the recording layer in responsev to the thermal pattern so as to produce a magnetic record of the image.

The subject invention also resides in apparatus for magnetically recording an image, comprising in combination a plurality of interdigitated first and second electrodes, a photoconductive layer in electrical contact with theinterdigitatedfirst and secondelectrodes, and a magnetic recording layerin heat-transfer relationship withthe photoconductivelayer. This apparatus further includes means connectedto the first and second electrodesfor electrically energizing the.photoconductive layer, means for exposing v the electrically energized, photoconductivelayer to the imageto produce a thermal pattern corresponding to the image, and means operatively associated with said magneticrecording layer for changing thestateof magnetization of the recording layer in response, to the thermal pattern whereby a magnetic, record of the image is established.

As already indicated above. in connection with a method aspect of the subject invention, the interdigitated first and secondelectrodes may, for example, be locatedwbetweenthe photoconductive layer and the magnetic recording layer or, alternatively, at least part of thephotoconductive layer may be located. between theinterdigitated first-and second electrodes and the magnetic. recording layer. in accordance with a preferred embodiment pursuant to the latter alternative version, the photoconductive layer mentioned in the preceding paragraph of this summarythas a first side lo catedgat thenamed plurality of interdigitated first and second electrodes, and has a second side opposite such first side.- The magnetic recording layer in this preferred, embodiment is in heat-transfer relationship with the photoconductive. layer through the latter second side. of the photoconductive layer.

The subject invention furtherresides in a composite recordingmedium, comprisingin combination a pluralityof'interdigitated firstand second electrodes, a photoconductive. layer in electrical contact with the interdigitated first and second electrodes, and a magnetic recording layer in heat-transfer relationship with the photoconductivelayer.

In accordance with a presently preferred embodiment of the subject invention, the photoconductive.

layer'mentioned in the, preceding paragraph has a first side located, at the namedplurality of interdigitated first and second electrodes, and has a second side opposite such first side. The magnetic recording layer is in this embodiment in heat-transfer relationship with the photoconductive layer through the latter second side of the photoconductive layer.

The above mentioned combination of photoconductive layer, interdigitated electrodes, and magnetic recording medium constitutes an'important feature of the subject invention, either by itself or in combination with other features.

To be sure, the use of interdigitated electrodes on photoconductors is old in the art of photoelectric devices. In the field of electrophotography, however, to a subgroup of which the subject invention belongs, the use of interdigitated electrodes on one side of the photoconductive layer has so far been generally rejected, and emphasis has been placed on the use of sandwich configurations in which the photoconductive layer is located between a pair of sheet-like electrodes.

In the course of intensive investigations into magnetic imaging we have, however, been able to reach the conclusion that the prior-art bias is unfounded and that an interdigitated electrode approach presents, indeed, the best overall solution as far as the particular field of magnetic imaging is concerned.

In particular, we have found that the interdigitated electrode structure acts in the manner of a halftone plate which materially improves image resolution and quality. We have also found that the interdigitated electrode structure greatly facilitates the premagnetization of the magnetic recording layer, which is important where information is recorded by a selective demagnetization of premagnetized material.

We have also found in the course of extensive tests that composite magnetic recording media of the type herein disclosed with interdigitated electrode structures displayless sensitivity to pin holes and lumps in the photoconductor. In sandwich-type photoconductive media, these irregularities lead to very annoying effects in the printed-out image.

We have further come to the conclusion that the heavy electric currents required for the thermomagnetic imaging effect require that the transparent top electrode in sandwich-type arrangements has to have a considerable thickness, which can lead to severe transmission losses in the incoming image. In contrast thereto, our interdigitated electrode structure does not impair light transmission in the areas between the conductors. This feature alone permits an increase in sensitivity by a factor of two or more, despite the fact that the conductors themselves obsure part of the incoming light.

An interdigitated electrode structure also permits a significantly improved management of the requisite current supply coupled with a very considerable reduction of instantaneous current flow, which has always been a problem in sandwich-type configurations. By way of example, if the photoconductor is exposed to the image within a narrow exposure band that extends across the interdigitated electrodes and that progresses during the exposure in a direction parallel to the electrodes, a very significant reduction of the current intensity required for the operation of the recording medium is accomplished, and a superior rheology is still preserved, since every light-exposed point is serviced by two adjacent electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description of preferred embodiments thereof, illustrated by way of example in the accompanying drawings, in which:

FIG. 1 is a perspective view of an imaging apparatus in accordance with a preferred embodiment of the subject invention;

FIG. 2 illustrates a first phase in the operation of the apparatus of FIG. 1; and

FIG. 3 illustrates further phases in the operation of the apparatus of FIG. I. 1

DESCRIPTION OF PREFERRED EMBODIMENTS The apparatus 10 of FIG. 1 includes a composite photoelectromagnetic recording medium 12 which embodies key features of the subject invention. The composite recording medium 12 includes a transparent substrate 13 which may be a sheet of glass or an organic equivalent thereof, such as a transparent hightemperature polyimide or polybenzamidazole.

An interdigitated electrode structure 14 including inter-digitated first and second electrodes 15 and 16 is preferably located on the transparent substrate 13, although the electrodes may alternatively be located on the photoconductive layer presently to be described. The deposition of electrode structures on a transparent substrate is a well-established art and does not as such form part of the subject invention. Suffice to say therefore, that the electrodestructure 14 may be deposited on the substrate 13 by evaporation, painting or sputtering, and that preferred electrode materials include gold, indium, chromium, and aluminum.

A photoconductive layer 18 is deposited on the substrate l3 and electrode structure 14. Suitable photoconductive materials include cadmium sulfide, cadmium selenide, alloys of cadmium sulfide or cadmium selenide, and sensitized zinc sulfide. High-gain photoconductor materials are presently preferred, since they provide for the large currents needed for heating of the magnetic material but require only moderate input light levels. The thickness of the layer 18 is not generally critical, but a layer thickness of about 3 to 10 microns is presently preferred for resolutions of more than about ten line pairs per millimeter.

The composite recording medium 12 further includes a magnetic recording layer 20 which is pressed against, coated on, or deposited on the photoconductive layer 18, to be in heat-transfer relationship therewith. Several materials and types of materials are suitable as a magnetic recording medium in the layer 20. Presently preferred materials for the magnetic recording layer 20 include low-Curie point media of the type described in US. Pat. No. 3,176,278, by L. J. Mayer, issued Mar. 30, I965, US. Pat. No. 3,250,636, by R. A. Wilferth, issued May 10, 1966, US. Pat. No. 3,368,209, by McGlauchlin et al., issued Feb. 6, 1968, US. Pat. No. 3,364,496, by Greiner et al., issued Jan. 16, I968, and British Patent Specification 1,189,232, filed Apr. 27, 2966 by E. I. du Pont de Nemours and Company. Presently preferred media include MnAs films and ferromagnetic chromium dioxide.

In the composite recording medium 12 we prefer the use of a magnetic recording medium 20 that has an acute temperature dependent transistion between the ferromagnetic and paramagnetic states at the Curie point. Where this is not inherent in the material, the preferred acuity may, according to the allowed US. Pat. application, Ser. No. 649,540, Magnetic Information Recording, filed June 28, 1967, by James U. Lemke, and assigned to the subject assignee, be realized by providing the low-Curie point magnetic particles with a shape anisotropy that dominates their crystal anisotropy.

Two preferred magnetic recording techniques include information-selective thermal demagnetization and information-selective thermoremanent magnetization. In selective demagnetization the recording medium 20 is typically premagnetized and is thereupon demagnetized by above-Curie point heating until the remaining magnetized areas present a record of the input image. In thermo-remanent magnetization a thermal pattern which represents the input image is imposed upon the typically unmagnetized recording medium 20 so that selected portions of that medium are heated above Curie point. These portions are thereupon cooled in the presence of a magnetic field whereby a thermoremanent magnetization of the type described by C. D. Mee, The Physics of Magnetic Recording (North-Holland Publishing Company, 1964), pp. 8084, and in the above mentioned Greiner et al US. Pat. and British du Pont Patent Specification takes place. If desired, strong positive magnetic records may be produced in the manner disclosed in Patent applications, Ser. Nos. 821,232 and 821,432, filed by Luc P. Benoit on May 2, 1969, and assigned to the subject assignee. As will become apparent in the further course of this disclosure, either of these magnetic recording techniques, or a combination of these techniques to be more fully described below may be employed for recording the magnetic image.

The apparatus further includes a source of light 22, such as an incandescent lamp or a fluorescent tube, which is energized from an electric power source 28 and which is combined with a reflector and shade 24 that defines a narrow longitudinal slit 25 for the emission of a sheet of light 26. The light sheet is reflected by a mirror 28 which is tiltableabout a longitudinal axis 29 and which relfects the impinging light in the form of a light sheet 30 onto a master record 31 that may, for instance, be supported on a sheet of glass (not shown).

The light 32 reflected by the master record 31 is imaged by a lens 34 onto the interdigitated electrode structure 14 and photoconductive layer 18.

The first and second electrodes and 16 of the inter-digitated electrode structure 14 are connected to opposite terminals of a source of electric energy 36. A switch 37 and a variable resistor 38 are connected in this circuit to control timing and intensity of the current flow to the electrode structure 14 and through the photoconductor layer 18.

If the magnetic record is to be established by thermoremanent magnetization, then the apparatus requires means for magnetizing portions of the magnetic recording layer 20 that, after an image-wise thermal exposure, cool down through the Curie point of the magnetic recording medium. 1f the magnetic record is to be established by selective demagnetization, then the apparatus 10 requires means for premagnetizing the magnetic recording layer 20 prior to its image-wise thermal exposure. The same elongated magnetizing head 40 may be employed for both methods with the difference that the magnetic field provided by the head 40 is applied to the magnetic recording layer 20 prior to image exposure if the record is to be established by selective demagnetization, and is applied after thermal exposure during the subsequent cooling step if the record is to be established by thermoremanent magnetization.

By way of example, it is assumed that the illustrated apparatus 10 operates with a combination of the two methods by imposing a premagnetization through the agency of thermo-remanent magnetization and by establishing the magnetic record by an image-wise selective demagnetization.

The magnetizing head 40 has an air gap 42, as well as an energizing winding 43 which is connected to a source of electric energy 44 through a switch 45 and a variable resistor 46 which serve to control the timing and intensity of the magnetic field 48 provided at the air gap 42.

To provide for a premagnetization, the photoconductive layer 18 and interdigitated electrode structure 14 are subjected to a uniform exposure. A flood lamp (not shown) which uniformly irradiates the electrode structure l4 and photoconductive layer 18 may be provided for this purpose. A light exposure which operates within a narrow exposure band 50 that extends across the alternating electrodes 15 and 16 and that progresses in the direction of an arrow 51 parallel to the electrode 15 and 16 is, however, preferred for two reasons. First, if the exposure is at any instant limited to a narrow exposure band, then the intensity of electric current flow is very substantially reduced relative to intensities that would occur if the entire composite medium were exposed at that instant. Secondly, it is easier to impose a premagnetization that has the requisite uniformity over the recording layer if such premagnetization takes place successively along a progressing band, rather than over the entire recording layer at once. Also, the total electric current flow is distributed over the electrodes if the exposure band extends across the electrodes rather than parallel thereto.

Accordingly, a light-reflecting element 53, only part of which has been shown, is initially substituted for the master record 31. The reflecting element 53, may, for instance, include a white sheet of paper or a mirror. The exposure band 50 is then swept across the composite recording medium 20 in the direction of the arrow 51 by a tilting of the mirror 28 about its axis 29. This tilting motion is effected by a drive 54 which is coupled to the mirror 28 and also to the magnetizing head 40 so as to move this head in the direction of an arrow 56 and in proportion to the tilting motion of the mirror 28, in such mutual synchronism that the magnetic field 48, which extends across the magnetic recording layer 20, follows the exposure band 50 in the direction of the arrow 51.

The switch 37 is closed and the variable resistor 38 is adjusted to supply sufficient electrical energy to the inter-digitated electrode structure 14 so that successive portions of the magnetic recording layer are heated above the Curie point of the magnetic recording medium, and do thereupon cool through the Curie point as the exposure band 50 travels along the arrow 51. The position of the traveling magnetizing head 40 relative to the moving exposure band 50 is selected such that the magnetic field 48, which extends in parallel to the exposure band 50, trails this exposure band at such a distance that the magnetic field 48 is successively present at all the magnetic layer portions that cool back through their Curie point.

In preparation of this thermoremanent magnetization, the switch 45 is closed and the variable resistor 46 is adjusted so that the magnetic field 48 has an intensity which is insufficient to magnetize the layer 20 while the same remains at a temperature below its Curie point, but which is sufficient to magnetize portions of the layer 20 that cool through their Curie point. As is known in magnetics, thermoremanent magnetization has the highest efficiency and linearity for all forms of magnetization.

The source 44 which energizes the magnetizing head 40 may be a source of direct current, in which case the thermoremanent premagnetization produces in the layer 20 a pattern of magnetized lines that extend in parallel to the electrodes and 16 and occupy regions that correspond to areas located between the interdigitated first and second electrodes. This magnetic line pattern effect is brought about by the fact that light exposure of the photoconductive layer 18 takes place between the inter-digitated electrodes 15 and 16 whereby areas below these electrodes remain colder than adjacent areas between these electrodes. To be sure, areas below the electrodes can be heated by thermal diffusion from adjacent regions, but a diffusion effect can easily be overcome by decreasing the light intensity of the source 22 or by increasing the traveling speed of the exposure region 50, or by a combination of these measures.

In contrast to a uniform premagnetization, a linepattem premagnetization provides a multitude of sharp magnetic gradients which promote large-area fill-in, increased contrast and in-creased resolution.

If a point-pattern, rather than a line-pattern, is desired, then the reflecting element 53 may be provided with a pattern of alternating light-reflecting and lightabsorbing areas, or the source 44 may be a source of current pulses, whereby every line 58 is only magnetized along spaced points, as shown at 60. Pointpatterns are sometimes preferred to line-patterns as is well known in the book and newspaper printing arts.

If the source 44 is a source of alternating current, each of the premagnetization lines 58 possesses a pattern of alternating magnetizations as shown in FIG. 2 at 62. In practice, this corresponds to a point-pattern magnetization, since no magnetizing effect takes place at and adjacent the zero crossovers of the alternating current. A strong toner attraction may result in this type of pattern from the alternating polarity of the magnetized points. 1

After the premagnetization step has been completed the reflecting element 53 is replaced by an information master record 31 that may, for instance, be a drawing, a writing or a printed text. By way of example, it is assumed in FIG. 1, that the master record 31 is a white sheet of paper on which a black character 65 has been printed.

For imaging according to the embodiment shown in FIG. 1, the switch 45 is opened and the magnetizing head 40 is decoupled from the drive 54 so that no magnetic field 48 follows the exposure band 50. The mirror 28 is, however, coupled to the drive 54 so that the light sheet 30 is moved across the master record 31 whereby successive portions of the master record are illuminated within a traveling narrow band 64. As before, the lens 34 images the band 64 onto the composite recording medium 12 whereby the latter is exposed within an exposure band 50 that travels in the direction of the arrow 51. The switch 37 is again closed and the variable resistor 38 adjusted so that electric currents in the photoconductive layer 18 heat the magnetic recording layer 20 above its Curie point at locations at which light reflected by the white background of the master record 31 impinges on the photoconductive layer 18. This above-Curie point heating causes a demagnetization of the recording layer 20 or of its premagnetized line or point pattern. An application of magnetic fields to the recording layer 20 is avoided at this stage to preclude an undesired remagnetization of demagnetized areas through the agency of thermoremanent magnetization.

No significant light is reflected by the black character 65, whereby portions of the photoconductive layer that correspond to that character remain dark. Since no electric currents are caused to flow at those corresponding portions, the temperature of the magnetic recording layer 20 remains below its Curie point within an outline occupied by an image of the character 65. The resulting lack of demagnetization of the premagnetized line or point-pattern leads to a magnetic record 68 of the character 65 as shown in FIG. 3.

A further substantial advantage of the apparatus, methods and media of the subject invention may be considered at this juncture. Information recording methods that rely on a selective demagnetization of a premagnetized medium are frequently hampered by the fact that areas which have been demagnetized are remagnetized by the magnetic information record itself when the recording medium cools back through its Curie point. Since this undesired remagnetization takes place through the agency of thermoremanent magnetization, it is strong and interferes with the resolution of the magnetic record.

With the composite recording medium of the subject invention it is easy to provide a super focusing effect since portions of the magnetic recording medium 20 corresponding to points of the photoconductive layer 18 located midway between adjacent electrodes 15 and 16 tend to acquire a higher temperature than areas corresponding to points located more closely to the elec trodes. The reason for this resides in the fact that the photoconductive layer portions below the electrodes 16 remain dark and do thus not produce heatgenerating electric currents, and that the electrodes themselves provide a heat-sink effect. Accordingly, the premagnetization lines 58, 60 or 62 can easily be made narrow and spaced from each other whereby a thermoremanent remagnetization effect brought about by the information record during cooling will limit itself to a broadening of the premagnetization lines or points until the same have dimensions desired for a printout of the magnetic image. This broadening is very beneficial since the thermoremanent magnetization effect under discussion typically provides magnetized fringes that have a polarity opposite to the magnetic polarity of the adjacent regions whereby strong magnetic gradients provide a sharp outline of the details of the magnetic image.

The magnetic image 68 may be stored in the magnetic recording medium 20 and printed out as often as desired. A magnetic toner 70 may be applied to the magnetic image 68 to render the same visible or printable. Magnetic toners typically comprise magnetically attractable particles 72 in powdered form or liquid suspension. Suitable materials for the toner particles 72 include iron, nickel, cobalt or ferromagnetic alloys. By way of example and not by way of limitation, preferred materials for the toner particles 72 include submicron particles of iron, carbonyl iron, and magnetite (Fe O Magnetic toners, toning techniques, and toning apparatus are disclosed in US Pat. No. 2,932,278, by J. C. Sims, issued Apr. 12, 1960; US. Pat. No. 3,052,564, by F.W. Kulesza, issued Sept. 4, 1962; and US. Pat. No. 3,250,636, by R.A. Wilferth, issued May 10, 1966.

The toner image 74 resulting from a toning of the magnetic image 68 may be printed out on a sheet of paper, on a film, or on another material having similar surface qualities. If desired, a sheet of paper 75 may be provided with an adhesive coating 76 to assure a transfer and retention of the toner image 74 in the form of a copy 78 of the original character 65. To effect such a transfer, the adhesive paper 75 is pressed onto the magnetic recording layer 20 an is thereupon removed whereby teh toner image 74 adheres to and is pulled off together with the paper 75. As is known in the art of magnetic printing, the toner 70 may be present in the form of a magnetic ink that is absorbed by the paper 75, or the toner particles may be provided with shells of a thermpolastic or other fusible material that may be fused to the paper 75 by a combination of pressure and heat. No adhesive coating 76 is provided if a magnetic ink or a fusible material is employed.

If no further printout or storage is desired, the magnetic image 68 may be erased by such conventional methods as an exposure of the magnetic recording layer 20 to anhysteretic magnetic erasing fields, or to above-Curie point heating. The latter may be effected by a sweeping of the light sheet 32 across the interdigitated electrode structure 14 and photoconductive layer 18, while the electrodes are connected to the electric energy source 36 and the reflective element 53 is substituted for the information record 31.

The magnetic recording layer 20 may be permanently deposited on the photoconductive layer 18. Since the magnetic recording layer 20 is typically much cheaper than the combined interdigitated electrode structure 14 and photoconductive layer 18, it may be advantageous to provide the magnetic recording layer 20 on a substrate that is selectively movable into contact with, and removable from, the photoconductive layer 18.

Reverting to the interdigitated electrode structure 14 it should be noted that the same affords a much better management of current supply than systems in which the photoconductive layer is sandwiched between sheet electrodes. In the inter-digitated structure shown in FIG. 1, any exposed point of the photoconductive layer is serviced by two current supply electrodes. If a light exposure shifts in the direction of the arrow 51, then an increase in electrical resistance along an electrode 15 is automatically compensated by a corresponding decrease in electrical resistance along an adjacent electrode 16. Electrode sheets, on the other hand, do not provide a similar favorable rheology so that contrast, resolution and quality of image points tend to be dependent on the location of the particular point relative to the edges of the recording medium.

We claim: 1. Apparatus for magnetically recording an image, comprising in combination:

a plurality of interdigitated first and second electrodes; a photoconductive layer in electrical contact with said interdigitated first and second electrodes; a magnetic recording layer in heat-transfer relationship with said photoconductive layer; means connected to said first and second electrodes for electrically energizing said photoconductive layer; means for exposing said electrically energized photoconductive layer to said image to provide in said recording layer a thermal pattern corresponding to said image; and means operatively associated with said magnetic recording layer for changing the state of magnetization of said recording layer in response to said thermal pattern whereby a magnetic record of said image is established. 2. Apparatus as claimed in claim 1, wherein: said photoconductive layer has a first side located at said plurality of interdigitated first and second electrodes, and has a second side opposite said first side; and said magnetic recording layer is in heat-transfer relationship with said photoconductive layer through said second side of said photoconductive layer. 3. Apparatus as claimed in claim 1, wherein: said means for changing said state of magnetization include means for premagnetizing said recording layer for subsequent image-wise demagnetization by said thermal pattern. 4. Apparatus as claimed in claim 3, wherein: said premagnetizing means include means for heating said recording layer above its Curie temperature and for subsequently magnetizing said recording layer while said recording layer cools through said Curie temperature. 5. Apparatus as claimed in claim 3, wherein: said premagnetizing means include means for magnetizing regions of said recording medium corresponding to areas between said interdigitated first and second electrodes. 6. Apparatus according to claim 1, wherein: said exposing means include means for progressively exposing said electrically energized photoconductive layer to said image in a direction in parallel to said interdigitated electrodes. 7. Apparatus according to claim 1, wherein: said exposing means include means for exposing said electrically energized photoconductive layer to said image within a band of exposure extending substantially across said interdigitated first and second electrodes and progressing in a direction parallel to said interdigitated electrodes. 

1. Apparatus for magnetically recording an image, comprising in combination: a plurality of interdigitated first and second electrodes; a photoconductive layer in electrical contact with said interdigitated first and second electrodes; a magnetic recording layer in heat-transfer relationship With said photoconductive layer; means connected to said first and second electrodes for electrically energizing said photoconductive layer; means for exposing said electrically energized photoconductive layer to said image to provide in said recording layer a thermal pattern corresponding to said image; and means operatively associated with said magnetic recording layer for changing the state of magnetization of said recording layer in response to said thermal pattern whereby a magnetic record of said image is established.
 2. Apparatus as claimed in claim 1, wherein: said photoconductive layer has a first side located at said plurality of interdigitated first and second electrodes, and has a second side opposite said first side; and said magnetic recording layer is in heat-transfer relationship with said photoconductive layer through said second side of said photoconductive layer.
 3. Apparatus as claimed in claim 1, wherein: said means for changing said state of magnetization include means for premagnetizing said recording layer for subsequent image-wise demagnetization by said thermal pattern.
 4. Apparatus as claimed in claim 3, wherein: said premagnetizing means include means for heating said recording layer above its Curie temperature and for subsequently magnetizing said recording layer while said recording layer cools through said Curie temperature.
 5. Apparatus as claimed in claim 3, wherein: said premagnetizing means include means for magnetizing regions of said recording medium corresponding to areas between said interdigitated first and second electrodes.
 6. Apparatus according to claim 1, wherein: said exposing means include means for progressively exposing said electrically energized photoconductive layer to said image in a direction in parallel to said interdigitated electrodes.
 7. Apparatus according to claim 1, wherein: said exposing means include means for exposing said electrically energized photoconductive layer to said image within a band of exposure extending substantially across said interdigitated first and second electrodes and progressing in a direction parallel to said interdigitated electrodes. 